124 DESCRIPTIONS OF PREPARATIONS. 



24. SHELL OF FRESH-WATER MUSSEL (Anodonta cygned). 



THE shell of Anodonta and of all Lamellibranchiaty is bivalve. The 

 two valves correspond to the two sides, right and left, of the body, and they 

 resemble one the other. The shell is therefore equivalve. Each valve 

 presents a short straight margin, the hinge-line, along which it is united to 

 its fellow, and which coincides very nearly with the whole of the dorsal 

 aspect of the animal, whilst the long curved margin coincides with its 

 ventral aspect. The external surface is marked by concentric lines parallel 

 with the margin and usually considered to mark distinct periods of growth 

 a point which cannot be regarded as certain. The areae inclosed by the 

 lines diminish progressively in size. The smallest area corresponds to the 

 original shell. This region of the shell is sometimes remarkably prominent 

 and is known as the beak or umbo. If a line be drawn to the ventral 

 margin from the centre of the umbonal region and perpendicularly to the 

 hinge-line, it divides each valve into a smaller anterior and a larger 

 posterior portion. Hence each valve is inequilateral. Those Lamelli- 

 branchiata which move from place to place by means of their distensible 

 foot, move invariably with the anterior part of the shell-valves foremost, 

 and never in the reverse direction. The shell of the Brachiopoda is also 

 bivalve : but it contrasts with that of Lamellibranchiata in several points : 

 the valves are dissimilar, i. e. the shell is inequivalve : they are divisible into 

 symmetrical right and left halves, i. e. are equilateral : and finally one valve 

 is dorsal, the other ventral according to the usual view. At any rate they 

 are not right and left as in Anodonta. The larger or ventral valve is 

 perforated by a peduncle of attachment, and is uppermost in the natural 

 position of the Brachiopod. 



The shell of Anodonta surpasses in size the shells of all other European 

 fluviatile bivalves. It also shows with remarkable distinctness the three 

 layers of which the Lamellibranch shell is usually composed the epicu- 

 ticula or periostracum, the prismatic layer and the layer of nacre or 

 mother-of-pearl. The epicuticula is purely organic and is composed of 

 conchiolin like the organic substratum of the rest of the shell. It is lamin- 

 ated, and the ridges on the outer surface of the shell are formed by it alone. 

 It constitutes the free border of the shell which is flexible in the natural 

 condition. The prismatic layer is calcified and is visible as a dark border 

 on the inner surface of the shell close to the margin. With a lens it has a 

 shagreen-like appearance due to its division into minute polygonal spaces. 

 The calcareous matter is in the form of more or less regular prismatic 

 needles. The nacre covers the whole inner surface of the shell within the 

 dark border mentioned above. It is iridescent in appearance owing to the 

 diffraction of light by the irregular free edges of the many delicate calca- 



